Are cloud forest tree structure and environment related in the Venezuelan Andes? TERESA SCHWARZKOPF, 1 * SUSAN J. RIHA, 2 TIMOTHY J. FAHEY 3 AND STEPHEN DEGLORIA 4 1 Instituto de Ciencias Ambientales y Ecológicas (ICAE), Facultad de Ciencias, Núcleo La Hechicera, Universidad de Los Andes, Mérida 5101,Venezuela (Email: teresas@ula.ve); and Departments of 2 Earth and Atmospheric Sciences and 3 Natural Resources and 4 Crop and Soil Sciences, Cornell University, Ithaca, NewYork, USA Abstract Cloud forest vegetation structure and composition were studied in theVenezuelan Andes at three sites in Mérida State. Although the sites are within 10 to 30 km of each other, climatic, geologic and topographic differences are remarkable. The main purpose of the study was to determine the relationship of specific environ- mental variables to forest vegetation characteristics, including basal area, tree height, density and diversity, and leaf area index (LAI). At 51 plots, all trees’ diameter at breast height >10 cm were recorded and identified. Although the environment at the three sites is distinctive, the tree species composition of the most abundant species was very similar. None of the measured environment variables were significantly correlated with the measured vegetation structure variables, except LAI, which was correlated with slope orientation; LAI showed higher values at south- facing plots.Tree height was relatively uniform, while basal area was highly variable and reached very high values. Stem densities were in the range reported elsewhere in cloud forests. Multivariate analysis using structure or composition data shows segregation of the plots by site. Principal component analyses by site indicate a minor impact of environmental factors on forest variables. At each site, a particular group of species are correlated with the ordination axes.We conclude that species pools and forest dynamics add to the complexity of the structure of the studied cloud forests. Key words: basal area, elevation, leaf area index, slope, topography. INTRODUCTION The presence of a belt of cloud forest vegetation on mountains in the tropics has been attributed to a sig- nificant local increase in water availability because of orographic precipitation and an increase in cloudiness (Grubb 1977; Bruijnzeel & Proctor 1995; Ataroff 2001), resulting in part from decreasing temperature along the elevation gradient. Less attention has been given to the variability in vegetation structure within elevation ranges of cloud forest belts. Although the presence of cloud forests is correlated with fog fre- quency rather than temperature (Grubb 1977; Cava- lier & Goldstein 1989), within the cloud forest belt, vegetation structure, especially tree height, has been related to various factors associated with elevation, like temperature, precipitation and relative humidity (Bruijnzeel & Veneklaas 1998; Ataroff 2001). However, some observations indicate a significant variability in forest structure not explained by eleva- tion only. Several studies on the impact of topography, fire, geology and succession on the variability of cloud forests (Haggar 1988; Guariguata 1990; Myster 1993; Takyu et al. 2002, 2003; Aiba et al. 2005; Martin et al. 2007) suggest complex forest–environment interac- tions and regional uniqueness. Moreover, the rela- tionships between cloud forest vegetation and environ- mental factors are still not well understood. In the Venezuelan Andes, high species diversity and the large number of endemic species (Kelly et al. 1994) have been attributed to topographic, geologic and climatic complexity (Soriano et al. 1999). Natural disturbances in Andean cloud forests are usually small in contrast to cloud forests in the Caribbean and Central America where hurricanes and fire are common. Neither the impact of disturbances nor abiotic factors on cloud forest tree structure have been addressed in Andean cloud forests. As topography is fairly complex in the Andean mountains, a high degree of variability in cloud forest structure can be expected. Although Nadkarni et al. (1995) suggest that similar environmental conditions do not imply similar forest structure or composition, it is expected that, at a smaller scale, topographic features impact cloud forest vegetation, as those affect temperature; stimulate *Corresponding author. Accepted for publication April 2010. Austral Ecology (2011) 36, 280–289 © 2010 The Authors doi:10.1111/j.1442-9993.2010.02160.x Journal compilation © 2010 Ecological Society of Australia